You indeed can change the Menu Item's Type to whatever you want, even after they have been created.

If, for instance, you want to change the Blog Section of a Menu link, go to the Control Panel->Menus Menu->[menuname]->Menu Item Manager and edit the Menu Item. Select the Change Type button and choose the new style of Menu Item Type from the available list. Thereafter, alter the Details and Parameters to reconfigure the display for the new selection as you require it.

Joomla! provides plenty of flexibility when displaying your Web content. Whether you are using Joomla! for a blog site, news or a Web site for a company, you'll find one or more content styles to showcase your information. You can also change the style of content dynamically depending on your preferences. Joomla! calls how a page is laid out a layout. Use the guide below to understand which layouts are available and how you might use them.

Content

Joomla! makes it extremely easy to add and display content. All content is placed where your mainbody tag in your template is located. There are three main types of layouts available in Joomla! and all of them can be customised via parameters. The display and parameters are set in the Menu Item used to display the content your working on. You create these layouts by creating a Menu Item and choosing how you want the content to display.

Blog Layout

Blog layout will show a listing of all Articles of the selected blog type (Section or Category) in the mainbody position of your template. It will give you the standard title, and Intro of each Article in that particular Category and/or Section. You can customise this layout via the use of the Preferences and Parameters, (See Article Parameters) this is done from the Menu not the Section Manager!